Clinical applications of ventricular assist devices to support patients with end-stage heart disease, as a bridge to cardiac transplantation, or as an end stage therapeutic modality have become an accepted clinical practice in cardiovascular medicine. It is estimated that greater than 35,000 persons suffering from end stage cardiac failure are candidates for cardiac support therapy.
Ventricular assist devices may utilize a blood pump for imparting momentum to a patient's blood thereby driving the blood to a higher pressure. One example of a ventricular assist device is a Left Ventricular Assist Device (LVAD). The LVAD is attached to the left ventricle of the patient's heart where oxygenated blood enters the LVAD through a blood inlet of the LVAD. The LVAD then imparts momentum to the blood. By connecting a blood outlet of the LVAD to the patient's aorta, pumped blood may reenter the patient's circulatory system.
Ventricular assist devices, such as the LVAD, have heretofore utilized positive displacement pumps and rotary pumps. Positive displacement pumps force blood from a first chamber to a second chamber by reducing the volume of the first chamber while increasing the volume of the second chamber to draw blood into the chamber. Such pumps are normally provided with check valves that only permit flow in one direction and are normally large and prone to mechanical wear. The human heart is a natural example of a positive displacement pump. A rotary pump forces blood by the spinning of an impeller within the pump. Known types of pumps utilize an impeller to impart momentum to the blood through the use of propeller type impeller blades which push the blood.
Rotary blood pumps may be either centrifugal or axial. In a centrifugal blood pump, blood enters the pump along its axis of rotation and exits the pump perpendicular to the axis of rotation. In an axial blood pump, blood enters the pump along its axis of rotation and exits the pump along the axis of rotation.
Traditionally, rotary blood pumps include a rotor consisting of a shaft and an impeller coupled to the shaft. Mechanical bearings are used to stabilize the rotor, both axially and radially, so the impeller could remain free to rotate smoothly while being constrained in the axial and radial directions. Mechanical bearings within the volume of blood have become the source of thrombosis. Moreover, as the use of mechanical bearings necessitated the protrusion of the shaft beyond the pumping chamber, a seal was required to prevent the escape of blood from the pumping chamber. This too became a source of thrombosis and sometimes homolysis, as well as premature wear.
The use of seals for mechanical shafts in rotary blood pumps has been shown to be suboptimal as seals could cause thrombosis of the blood and could wear out prematurely. To minimize the risk of thrombosis and failed seals, sealless rotary blood pumps have been developed. For example, U.S. Pat. No. 5,695,471 to Wampler and U.S. Pat. No. 6,846,168 to Davis et al. (the '168 patent), both herein incorporated by reference, relate to sealless rotary blood pumps. In such sealless rotary blood pumps, the rotor and/or impeller may be suspended within the pumping chamber by the use of magnetic and/or fluid forces.
Magnetic and/or fluid forces used to suspend the impeller within the pumping chamber could serve to stabilize the impeller, allowing for rotation while preventing excessive axial or radial movement. Wearless stabilization of an impeller can be achieved by magnetic bearings and hydrodynamic bearings. In this way, magnetic forces form magnetic bearings and fluid forces form hydrodynamic bearings.
Several forms of magnetic bearings have been developed. In one form, passive magnetic bearings in the form of permanent magnets can be embedded in both the rotor and the pump housing to provide magnetic coupling that may keep the impeller suspended in position within the pump casing. Such permanent magnets embedded in both the rotor and the pump casing provide repulsive forces that may keep the impeller suspended within the pump casing. Such magnetic bearings are said to be passive magnetic bearings as no control is used to keep the impeller properly centered. While passive magnetic bearings may be effective at keeping the impeller suspended in one direction, for example in the radial direction, it has been shown that such passive magnetic bearings alone cannot keep an impeller suspended in both the axial and radial directions.
Active magnetic bearings in the form of electromagnets can be used, for example in or on the pump housing, magnetically to couple with and to drive the impeller. Power to the electromagnets may then be varied, as required, to adjust the magnetic field in response to displacement so that the impeller may be kept in position.
Electromagnets may also be used, for example, in the pump casing, to provide the repulsive magnetic force. These bearings are said to be active magnetic bearings as the magnetic fields are actively controlled to maintain proper impeller position.
Because of the complexity of active magnetic bearings, rotary blood pumps have been developed to use both passive magnetic bearings and hydrodynamic bearings to suspend the impeller in a sealless rotary blood pump. For example, U.S. Pat. No. 6,234,772, to Wampler et al. (the '772 patent), herein incorporated by reference, relates to a sealless rotary blood pump with passive magnetic bearings and hydrodynamic bearings. In the '772 patent, radial suspension is enabled by a series of magnetic discs within the impeller shaft and corresponding series of magnetic rings in the pump casing. In the '168 patent, radial suspension is enabled by a series of magnetic rings within a spindle that protrudes through a hole in the center of the impeller. A corresponding series of magnetic discs is provided within the impeller whereby the impeller is suspended about the spindle during rotation. In the '772 patent, axial suspension is enabled by a set of hydrodynamic thrust bearing surfaces on the impeller.
There remains a need for smaller and more efficient rotary blood pumps. In particular, there remains a need for wearless centrifugal pumps with hydrodynamic bearings and improved continuous fluid flow paths within the pump to further diminish the risks of hemolysis and thrombosis in the blood being pumped. By developing more sophisticated rotary blood pump impellers with hydrodynamic bearings and passive magnetic bearings, the physical size, performance and efficiency of the rotary blood pump may be improved to the point where consistent and reliable therapeutic support may be provided.